This invention relates generally to wristwatch movements incorporating stepping motors of the Lavet type, which drive the hands of the wristwatch through a gear train. The motor is of the type which receives periodic stepping pulses from a quartz-synchronized integrated circuit. More particularly, the invention relates to the assembly method of manufacture of the bipolar permanent magnet rotor member with drive pinion for the stepping motor.
Wristwatch movements are known which drive a center wheel directly connected to the minute hand by means of a stepping motor rotor acting at the periphery of a wheel connected to the minute hand shaft. By this means, a single reduction gear member can perform the proper reduction to drive an hour hand shaft coaxial with the minute hand shaft. Such an arrangement is shown in U.S. Pat. No. 4,376,996 issued Mar. 15, 1983, in the name of the present inventor and assigned to the present assignee. In a copending application Ser. No. 835,673 filed Apr. 14, 1986, also in the name of the present inventor and assigned to the present assignee, now U.S. Pat. No. 4,647,218 issued Mar. 3, 1987, a timepiece stepping motor of the Lavet-type includes a rotor with a pair of driving pins stepping the rotor 180 degrees once each minute. The pins drive a toothed wheel directly at its periphery which is also connected to the minute hand, so as to step the minute hand once each minute.
Another copending application, Ser. No. 843,020 filed Mar. 24, 1986, now abandoned and filed Dec. 22, 1986 as continuation-in-part application Ser. No. 945,268 in the name of the present inventor and assigned to the present assignee disclosed a two piece stator and frame plate assembly for a stepping motor having a rotor with a pair of driving pins serving as the motor drive pinion.
Constructions are known in which a bipolar permanent magnet serving as the rotor in a two-piece stator of a Lavet stepping motor includes an integral toothed driving pinion, which cooperates with a toothed center wheel for a timepiece. The arresting mechanism for holding the rotor magnet at rest position with respect to the stator between steps is formed by interaction between the ends of the stator poles and the polarity of the permanent magnet rotor.
Another construction, in which a bipolar permanent magnet rotor is provided in a Lavet stepping motor with a one-piece stator is shown in U.S. Pat. No. 4,571,516 issued Feb. 8, 1986. There, a rotor magnet with flat pole ends is inserted into a recess in a plastic rotor body and glued or riveted into place. The magnet has flat ends which interact with flats on the stator opening to determine the stable or rest position.
Known constructions in which the permanent magnet rotor incorporates two driving pins or lugs attached to a central shank passing through the permanent magnet of the stepping motor rotor are shown in U.S. Pat. No. 4,483,627 issued Nov. 20, 1984 and in PCT application No. EP 79/00025 filed Mar. 30, 1979 and published Nov. 15, 1979 under No. WO/9/00930.
If the permanent magnet is round, it is difficult to accurately determine the axis of magnetization which causes the magnet to cooperate with the stator to determine its rest or stable position. This makes it very difficult to provide the proper offset angle between the axis of magnetization and the axis passing through the driving pinion teeth as they make engagement with the toothed wheel.
A better utilization and alignment with the axis of magnetization can be achieved by providing flats on the sides of the permanent magnet as in U.S. Pat. Nos. 3,969,642 and 4,571,516, which also allow the space around the flats to include some plastic material in the case of U.S. Pat. No. 4,571,516. However, this does not solve the problem of properly orienting the axis of magnetization with respect to the axis of engagement passing through the drive pins.
According, one object of the present invention is to provide an improved bipolar stepping motor rotor with drive pinion which properly orients the pinon with respect to the bipolar permanent magnet.
Another object of the invention is to provide an improved method of manufacture of a bipolar stepping motor rotor with drive pinion.
Still another object of the invention is to provide an improved stepping motor rotor for a Lavet stepping motor directly driving a center wheel.